1. Field of the Invention
The present invention relates to novel vapor sampler apparatus useful in preparing sensitive, hand-held explosives detector systems. In particular, it addresses the problem of sampling explosives vapors, i.e., obtaining the vapor sample and transporting it to the detector in toto, without significant loss. The sampler apparatus are useful in sampling for any electronegative explosive vapor, including those such as plastic explosives, which have very low vapor pressures.
2. Description of Related Art
A major problem associated with the detection of explosives in airports, nuclear facilities, secure areas, and elsewhere is that the molecules which are available for detection are available in only minute quantities. For example, in the case of the lower vapor pressure explosives, there are only about 6.times.10.sup.-14 gm of explosives molecules per cubic centimeter (cc) of air at room temperature and pressure. This is approximately 1 million times less than the amount of pollution regularly found in a cc of air on a good day. Thus, the exceedingly low quantity of explosives molecules available for detection indicates the need for a highly sensitive detector system.
For many years, Electron Capture Detectors (ECD) have been employed commercially in explosives detection systems, and have been thought to be the detector of choice in such systems. An ECD is normally used in line in an explosives detection system after the materials present in a test sample have been separated on a gas chromatography column. Such a detection system is generally denoted as a GC-ECD System. While the sensitivity number for an ECD detector has recently been calculated to be 8.times.10.sup.-15 gm of explosives/cc, most commercial GC-ECD detection devices cannot consistently detect trinitrotoluene (TNT) vapor at 6.times.10.sup.-11 gm of explosives/cc. One possible reason for the loss of sensitivity encountered when using a GC-ECD detection system is that explosives molecules possess adhesive properties, sticking to every metal, plastic, ceramic, etc. with which they have been tested. It is this property that excludes the commercial GC-ECD devices from detecting all the explosives of interest since many of the explosives molecules in a test sample stick to surfaces of the instrument which precede the ECD itself, and thus never reach the detector.
The results of tests on a variety of commercially available explosives detectors, involving evaluation of their sensitivity to a variety of explosives, identification of false alarm agents, and general performance and maintenance characteristics, have revealed that most explosives detectors have difficulty in detecting, or are not applicable to, the detection of RDX (cyclonite) or PETN (pentaerythritol-tetranitrate) plastic explosives, or any other electronegative explosives which have vapor pressures in the low range of these materials (i.e., about one part per trillion).
Bradshaw et al., in U.S. Pat. No. 3,998,101, disclose a device for sampling non-hermetically sealed containers, but do not specify the detector employed. With the disclosed system, when the sample is taken, it is removed and transported to the detector. The system is not "hand held", and in addition, the power requirements for the sample portion of the device are quite large.
Reid et al., in U.S. Pat. No. 4,718,268, disclose an explosives detection device wherein the sample is obtained and subsequently transported to the detector. The detector is a mass spectrometer which weighs several hundred pounds, and thus is not "hand held". This system is designed for sampling large volumes.
Griffiths, in U.S. Pat. No. 4,818,870, discloses an explosives detection device which is a sampling probe only, and not a sampling/detection system. The size of the probe described therein indicates that it is not intended as a "hand held" device. Bather, in U.S. Pat. No. 4,820,920, discloses an explosives detection system which is neither "hand held" nor "portable".
Finally, Lanning et al (Anal. Chem. (1988) 60: 1994-1996) have described a detector device which differs from that of the present invention in that the "sampler" does not move into the external environment to collect the sample. Failure to extend the sampler out into the area to be screened accounts for the inability of commercial explosives detectors to detect all of the explosives present in a sampling environment.